Cartilage, which provides the cushioning surface between joints, undergoes age-associated changes including loss of chondrocytes, the sole cell type in cartilage. The main goal of this project is to establish the feasibility of tissue engineering of cartilage. In this project, adult rabbit and aged human articular chondrocytes are phenotypically modulated and expanded in vitro with growth factors. These cells retain the capacity to form hyaline cartilage when re-introduced into animals. These "secondary chondroprogenitor cells" may be useful for cell-based repair of cartilage defects. An immortalized chondrocyte line developed in this laboratory displays a similar pattern of matrix metalloproteinase expression upon cytokine stimulation as has been described in human osteoarthritis. This cell line will be useful for studying the mechanisms involved in regulating MMP expression and activation.